<!doctype html>
<meta charset="utf-8">
<style>
  .flex {
    display: flex;
    width: 100px;
    border: 1px solid;
  }
</style>
<div class="flex">
  <div>
    <div style="background: green; width: 150px; height: 50px;"></div>
  </div>
</div>
